An electric lamp comprising an amber glass envelope is described in patent EP1193226. The colored glass has a formula of R′2O—RO—SiO2, wherein R′ is an alkali metal element and R is an alkaline earth metal element. The colored glass comprises molybdenum (Mo) and sulfur (S), which together give the amber color to the glass. The molybdenum is added in the proportion of 0.01 to 0.6% of weight (expressed in the oxidic form) and the sulfur is added in the proportion of 0.01 to 1% of weight (expressed in the oxidic form). Although the proportion of molybdenum and sulfur in the obtained glass is not given in this patent, the applicant has noted that an amber glass suitable for signal lighting may comprise between 0.01 and 1% by weight of molybdenum expressed in oxidic form and between 0.01 and 2.5% by weight of sulfur expressed in oxidic form. It is to be noted that it is conventional in the field of glass science to express the proportion of a constituent as the oxidic form. Hence the percentage of Mo is expressed as the weight percentage of MoO3 and the percentage of S is expressed as the weight percentage of SO3. However, this does not mean that this constituent is only present in the glass under this oxidic form. The constituent may be present in various forms, and it is even possible, for instance, that the glass does not contain the oxidic form of the element.
However, the amber glass described in this patent appears to be fragile, which is a drawback because an electric lamp made with this glass may easily be broken during operation or even before it is used in a dedicated automotive application.